Micrite Cement Research Articles

Microbial mediated hardening of a Central Mediterranean upper shelf seafloor during the MIS 5.5; a possible post global warming scenario?

Higher sea level and sea surface temperature than at present have been inferred for the Last Interglacial (MIS 5.5, 135–116 ka), making it a good analogue for modeling the climate response of the environment to global warming in the near future. With the aim of predicting the possible evolution of some central Mediterranean shallow sandy seabeds, a MIS 5.5 biocalcarenite was investigated (Gulf of Taranto, Italy). Limited post-depositional diagenesis affected this deposit, which represents an infralittoral sandy seabed, with local vegetation and relatively high energy conditions. Medium-coarse sand sized bioclasts, with a negligible quantity of siliciclastic, compose the sediment that also hosts large shells of mollusks. Micritic cements are widespread, often in continuum with the micritized part of the shells, showing non-isopachous aphanitic and filamentous rims, aphanitic micro-mounds, vacuolar peloidal menisci, and aphanitic pore-filling matrix. All these cements consist of submicrometer anhedral or nanospheroidal crystals of low-Mg calcite, mixed with a smaller amount of irregular plate-like crystals of saponite. Micritic cements are also rich in mineralized filamentous, tubular, and subspherical bacterial bodies. This highlights the occurrence of an epilithic and endolithic microbial community forming a biofilm that stabilized the mobile sediment as consequence of the microbial mediated early cement precipitation. This led the synsedimentary hardening of some parts the mobile sandy substrate, the settlement of sessile taxa - such as Spondylus gaederopus, oysters, serpulids and barnacles, together with endofaunal organisms. Early micritic cementation is common in modern tropical climate, whereas is substantially absent in the modern Mediterranean. Consequently, its presence in the MIS 5.5 deposit confirms warmer sea water temperature compared to today (estimated at ca +2 °C) and suggests that similar hardening of mobile substrates may occur in the near future as a response to global warming.

Serpulid microbialitic bioherms from the upper Sarmatian (Middle Miocene) of the central Paratethys Sea (NW Hungary) – witnesses of a microbial sea

Abstract. We present previously unknown stacked bowl-shaped bioherms reaching a size of 45 cm in diameter and 40 cm in height from weakly solidified peloidal sand from the upper Sarmatian of the Paratethys Sea. The bioherms were mostly embedded in sediment, and the “growth stages” reflect a reaction on sediment accretion and sinking into the soft sediment. The bioherms are spirorbid–microclot–acicular cement boundstones with densely packed Janua tubes surrounded by microclots and acicular cement solidifying the bioherm. The surrounding sediment is a thrombolite made of peloids and polylobate particles (mesoclots) which are solidified synsedimentarily by micrite cement and dog-tooth cement in a later stage. The shape of the bioherms reflects a series of growth stages with an initial stage (“start-up stage”) followed by a more massive “keep-up stage” which grades into a structure with a collar-like outer rim and a central protrusion and finally by a termination of growth (“give-up stage”). The setting was a shallow subtidal environment with normal marine or elevated saline, probably oligotrophic, conditions with an elevated alkalinity. The stacked bowl-shaped microbialites are a unique feature that has so far been undescribed. Modern and Neogene microbialite occurrences are not direct analogues to the described structures, but the marine examples, like in The Bahamas, Shark Bay and the Persian Gulf, offer insight into their microbial composition and environmental parameters. The microbialites and the surrounding sediment document a predominance of microbial activity in the shallow marine environments of the Paratethys Sea during the late Middle Miocene, which was characterized by a warm, arid climate.

Indigenous microbial communities as catalysts for early marine cements: An in vitro study

AbstractEarly marine cementation is a fundamental process for many characteristics of carbonates, like the stabilisation of steep slopes. The genesis of early cements is often attributed to physicochemical processes but there is evidence for microbial mediation. To elucidate the role of microbes and associated organic material, in vitro experiments were undertaken in the presence and absence of indigenous microbiota in ooids from Schooner Cays, Bahamas and compared with native grapestones from Joulter Cays, Bahamas. Microscopic examinations by stereomicroscopy, scanning electron microscopy and thin section analysis of in vitro incubations with native flora document rapid grain fusion, resulting in the formation of grapestones within 30–60 days. The initial binding of the grains is primarily facilitated by exudates of extracellular polymeric substances and microbial communities acting as catalysts in the formation of micritic bridges, cements and encrusted aggregates. In vitro grapestones are similar to native grapestones from Joulter Cays with intergranular areas infested with extracellular polymeric substances, microbes, micritic cements, amorphous calcium carbonate nanograins and micritised outer surfaces. These similarities suggest that incubations with native flora follow similar mineralisation mechanisms as in the natural environment. In contrast, sterilised grains remain loose with little crystal formation after 60 days and are devoid of microbes and organic exudates. Owing to the near absence of precipitates, abiotic precipitation is not the driving force promoting early cements. In contrast, grain fusion is microbially mediated via both a passive mechanism, where extracellular polymeric substances and cell surfaces function as templates for crystal nucleation and generation of micritic cements, and through an active mechanism by which biofilm heterotrophs and autotrophs induce chemical alterations of a local environment, facilitating precipitation. This study underscores that microbially mediated cementation can occur at fast rates and that firmground to hardgrounds and slope stabilisation take place shortly after deposition of carbonate grains.

Guttulatic calcite: A carbonate microtexture that reveals frigid formation conditions

Abstract The paragenesis of carbonate pseudomorphic textures in the rock record that are inferred to represent replaced metastable ikaite (CaCO3·6H2O), which forms at frigid temperatures, is uncertain. Petrographic analysis of Mono Lake (California, USA) Pleistocene tufas allowed recognition of a distinctive calcite microtexture, termed guttulatic calcite, that forms during carbonate dehydration and is diagnostic for precursor ikaite. The texture is characterized by pseudo-hexagonal or spherical low-Mg cores, which likely formed initially as vaterite, with an ellipsoidal overgrowth, and a secondary high-Mg sparry or micritic cement. Observations of Mono Lake ikaite pseudomorphs, combined with a review of more ancient examples, indicate that guttulatic texture records carbonate dehydration of precursor ikaite and can be used to infer frigid paleotemperatures.

Discovery of enigmatic toroidal carbonate concretions on the Rio Grande Rise (Southwestern Atlantic Ocean)

We present enigmatic toroidal carbonate concretions retrieved from 700 m water depth from two sites in the upper plateau of the Rio Grande Rise. The concretions have a diameter of ~15 cm and a central hole of ~5 cm, and were observed on top of loose bioclastic sand over an area of ~30 m2 at 0.5–1.5 m from one another. They consist of brown, porous, bioclastic grainstone, lacking internal structures. Grains consist of sand (< 3% coarse, 30% medium, 35% fine, 25% very fine), composed mainly by planktonic foraminiferal tests, and < 10% lime mud. The observed foraminiferal species indicate initial deposition of the sand in an open ocean setting. Biostratigraphy suggests an age no older than Pleistocene. Petrographic thin sections and SEM reveal that the fossiliferous grainstone contains intraclastic micritic cement and isopachous rim cement made of bladed magnesian calcite. δ18O values range from +1.5 to +3.3‰ (V-PDB) and increase with the degree of cementation, while δ13C ranges from +0.5 to +2.3‰ irrespective of cementation. The cementation of the grainstones is likely to have taken place in the marine phreatic environment. Carbonate precipitation induced by methane oxidation or (subaerial) meteoric diagenesis are ruled out based on both cement fabric and isotopic composition. Plausible causes for the toroidal shape of these structures could be: 1) sediment excavation by organisms, or 2) cementation within biofilms around burrows, followed by selective seafloor erosion. However, unveiling the actual formation mechanisms warrants further investigation.

Beachrock as a Paleoshoreline Indicator: Example from Wadi Al-Hamd, South Al-Wajh, Saudi Arabia

The present study concerns the Holocene inland beachrocks that are exposed in the Red Sea coastal plain at the mouth of Wadi Al-Hamd, South Al-Wajh City, Saudi Arabia, and their utility as an indicator for Holocene climate and sea level changes. In addition, the framework composition, and carbon and oxygen isotopic data, are employed to interpret the origin of their cement. The beachrock consists mainly of gravel and coarse-grained terrigenous sediments dominated by lithic fragments of volcanic rocks, cherts and rare limestones along with quartz, feldspars and traces of amphiboles and heavy minerals. In addition, rare skeletal remains dominated by coralline algae, benthic foraminifera and mollusca remains are recognized. The allochems are cemented by high Mg-calcite (HMC) formed mainly in the intertidal zone under active marine phreatic conditions. The cement takes the form of isopachous to anisopachous rinds of bladed crystals, micritic rim non-selectively surrounding siliciclastic and skeletal remains, and pore-filling micrite. Pore-filling micrite cement occasionally displays a meniscus fabric, suggesting a vadose environment. The δ18O and δ13C values of carbonate cement range from −0.35‰ to 1‰ (mean 0.25‰) and −0.09‰ to 3.03‰ (mean 1.85‰), respectively, which are compatible with precipitation from marine waters. The slight depletion in δ18O and δ13C values in the proximal sample may suggest a slight meteoric contribution.

Microbial contribution to early marine cementation

AbstractThe origin of early marine cements is still being debated as either abiotic, triggered by evaporation and CO2 degassing, or microbially‐mediated. In particular, micritic envelopes and micritic meniscus cements have been proposed to be microbially‐mediated but the microbial community and the resulting organomineralization processes have rarely been documented. Herein, scanning electron microscopy and thin section analyses of samples from the Bahamas and Hamelin Pool, Australia, provide evidence that early cements in intergranular areas are the result of organomineralization processes driven by both active (microbial metabolism) and passive mechanisms influenced by mucilaginous extracellular polymeric substances and/or microbial cell walls acting as matrices for crystal nucleation and growth. Signatures of biogenicity are supported by the conspicuous presence of early micritic cements in association with dense filamentous fabrics and entombed microbial forms. Most importantly, the prolific colonization of metabolically active communities, many of which assist in carbonate precipitation, trapping, binding and encrustation of the grains, testifies to the contribution of microbes in early cementation processes. The extent and ubiquity of mucilaginous extracellular polymeric substances and its concomitant association with amorphous calcium carbonate nanograins indicates that passive organomineralization via an intermediary amorphous phase is the other leading process for the generation of micrite and fine mesh of aragonite needle‐like crystals. Furthermore, the conspicuous occurrence of microbially‐mediated micrite, meniscus and bridging cements, as well as microcrystalline Mg‐calcite cement in the marine phreatic realm challenges the widespread notion that these fabrics are indicative of marine vadose or quiet marine phreatic environments. All analyzed samples show a remarkable similarity in early cement fabrics and a consistent superposition of prismatic cements on the micrite meniscus and bridging cements. These microbially‐mediated micritic cements represent the first generation of marine cements upon which aragonite and high‐Mg calcite develop.

A Thermal Pulse Induced by a Permian Mantle Plume in the Tarim Basin, Northwest China: Constraints From Clumped Isotope Thermometry and In Situ Calcite U‐Pb Dating

AbstractHigh burial temperature and a lack of material for traditional thermal proxies significantly impede the understanding of the thermal history of carbonate stratigraphic sequences in deep time. Carbonate clumped isotope thermometry interpreted through models of solid‐state isotopic reordering and in situ U‐Pb dating provides a new methodology for reconstructing thermal histories of such successions. Calcite fabrics (micrite cements and veins) from deeply buried (5,000–8,000 m) Ordovician carbonate intervals in the Tarim Basin were analyzed for their carbonate clumped isotope compositions and U‐Pb ages, and the results were used to estimate maximum peak burial temperatures. Reconstructed burial temperatures reveal a previously ignored thermal pulse in the range of 160°C–190°C during the Permian period. Maximum geothermal gradients at the time of peak burial temperature varied from 26.8°C/km to 69.3°C/km, with higher values in the northern Tarim Basin. We show that the spatial patterns of peak burial temperature and geothermal gradients closely correspond to the distribution of the Tarim Large Igneous Province, which emplaced during the Permian following the emplacement of a mantle plume beneath the Tarim Basin. We thus propose that the thermal pulse recorded in Tarim Ordovician limestones resulted from Permian mantle plume activity. The reconstructed thermal history has important implications for our understanding of source‐rock hydrocarbon generating history, hydrocarbon accumulation, and fluid flow history in the Tarim Basin. This study also demonstrates the potential of the integration of Δ47, U‐Pb dating of carbonates and solid‐state reordering models as an integrated thermochronological method for reconstructing thermal histories of deeply buried carbonate intervals.

RESERVOIR PROPERTIES OF BARREMIAN–APTIAN URGONIAN LIMESTONES, SE FRANCE, PART 2: INFLUENCE OF DIAGENESIS AND FRACTURING

Integrated sedimentological, diagenetic and structural analyses have been carried out on microporous and tight Urgonian (Barremian – Aptian) limestones in a study area in SE France in order to understand the influence of diagenetic changes and structural deformation on the spatial distribution of reservoir properties. A diagenetic history for the carbonates was established and was divided into phases which correspond to episodes of regional geodynamic activity. Petrographic (optical, SEM and cathodoluminescence microscopy), structural and geochemical (δ18O, δ13C) studies were carried out to characterize the cement phases in the carbonates, especially micrite and blocky calcite, and to investigate their relationship with episodes of fracturing.Eleven calcite cement phases and four micritic cement phases were identified in relation to the two main phases of structural deformation which affected the Urgonian limestones. A first phase of micrite cementation occurred early in the diagenetic history and was linked to early marine cementation at the tops and bases of depositional cycles during the Barremian. A major phase of micrite recrystallization, which generated microporosity in carbonates that had previously been preserved from early cementation, was followed by a first phase of blocky calcite which occluded intergranular pore spaces. The blocky cement formed in a shallow burial meteoric environment and contributed to the preservation of microporosity during late Durancian tectonism (Albian – Cenomanian). A second phase of blocky calcite is associated with fracture activation during latest Eocene (Priabonian) – Oligo‐Miocene extension.Reservoir rock‐types (RRTs) proposed in a previous study were consistent with the diagenetic characteristics and the results of δ13C / δ18O analyses. Microporous RRTs formed as a result of early to late shallow burial processes and display low δ13C values; whereas cemented RRTs developed both due to early marine cementation (with high δ13C values) and/or as a result of cementation related to fluid flow linked to the reactivation of faults and fractures. This suggests that some late diagenetic and microstructural processes were pre‐determined by early diagenetic changes in the carbonates. The resulting stratigraphic architecture consists of a vertical stacking of weakly fractured microporous limestone intervals alternating with highly fractured, cemented limestone units.

Trends in Stable Isotopes and Climate Proxies From Late Changhsingian Ghost Landscapes of the Karoo Basin, South Africa

The stable carbon- and oxygen-isotope values derived from in situ pedogenic carbonate-cemented nodules and vertebrate apatite in the Daptocephalus and overlying Lystrosaurus Assemblage Zones of the Balfour Formation, Karoo Basin, South Africa, have formed the basis for previous interpretations of a unidirectional climate trend toward hyper-aridity across the biozone boundary. This assemblage-zone boundary has been considered by many authors to be equivalent to the Permian–Triassic boundary in the basin. To better understand the climate under which these vertebrate assemblages existed, we have analyzed the carbon- and oxygen-stable isotopes of pedogenic carbonate nodules sampled from fourteen horizons of intraformational pedogenic nodular conglomerate (PNC) at Old Lootsberg Pass, a classic locality at which the Permian–Triassic boundary is reported. Analysis of these refractory soil constituents provides insight into the climate under which these “ghost” soils formed, where no other physical record of their existence is found in the stratigraphy. A positive correlation between δ13CVPDB and δ18OVSMOW values of micrite cements is defined by analyses of carbonate nodules taken from a measured stratigraphic thickness of ∼200 m, which spans the biozone boundary as currently defined. For samples taken from the same lag deposit, similar and relatively narrow ranges of isotope values are encountered. Samples cluster into two isotopic groups. The values in the first group cluster more tightly in all sampled nodules (δ13CVPDB −2.3 to −6.5‰; δ18OVSMOW 13.8–15.1‰), and are interpreted to indicate that these originated from paleosols that formed under similar climate controls. Values from the second sample group display a wider variance between analyses (δ13CVPDB −5.2 to 14.0‰; δ18OVSMOW 8.8–15.5‰). These nodules are interpreted to indicate that they originated under polygenetic soil-forming conditions representing the reworking of either: (1) more than one paleosol, the calcite-cemented nodules of which represent precipitation under both closed and open-system controls; or (2) one or more compound-composite paleosols. Stable-isotope trends based on PNCs analyzed, thus far, demonstrate an overall shift over time in the ghost landscapes. More seasonally dry soils formed under a climate that can be characterized as warm/dry accompanied by lower precipitation in the lower part of the section. In contrast, soils in the upper part of the section formed under cool and moist conditions, with increased precipitation near the biozone boundary. Hence, latest Permian climate associated with the more seasonally dry landscapes demonstrate a trend toward cooler and wetter conditions, which is opposite to the trend widely held in the literature.

Variation in Saturated Hydraulic Conductivity at the Outcrop Scale, the Whanganui Basin, New Zealand.

Groundwater flow and contaminant transport are strongly influenced by hydrogeological spatial variation. Understanding the textural heterogeneity of aquifer and aquitard units is critical for predicting preferential flow pathways, but is often hindered by sparse hydrogeological data, widely spaced data points, and complex stratigraphy. Here, we demonstrate the application of a relatively new air permeameter technology, providing a cost-effective, rapid alternative for characterizing hydrostratigraphic units in the field. The aim of this research is to (1) characterize the variation of saturated hydraulic conductivity across shallow-marine hydrostratigraphic units of the Whanganui Basin, New Zealand, and (2) assess the variation of saturated hydraulic conductivity within individual hydrostratigraphic units and relate these changes to facies and depositional environments. Results suggest heterogeneity within fine-grained aquitard units is controlled by bioturbation, whereby burrowing, ingestion and defecation results in grain size segregation and differential micrite cementation. Coarse-grained heterolithic aquifer facies display sharp changes in permeability across planar to cross-bedded sets, related to current and wave energy fluctuations within shallow-marine depositional settings. Bedding plane orientation creates high permeability zones that promotes down dip subsurface flow. Down dip gradation of coarse-grained nearshore facies into fine-grained shelf facies along the paleo shoreline-shelf transect is suggested to promote lateral and vertical groundwater flow within the basin fill. Air permeameter techniques have potential for application within groundwater basins around the world, providing datasets that facilitate greater understanding of groundwater systems, informing practices and policies for targeted water quality management.

Beachrock Cementation Patterns Along the Gulf of Aqaba Coast, Saudi Arabia

Beachrocks crop out along the Saudi coast of the Gulf of Aqaba with beds lying at elevations up to 0.8 m above the present sea level and with seaward extents that repeatedly submerge and emerge during high and low water levels. This study discusses the cement composition, petrography and bedding features of beachrocks at 24 different locations, of which a total of eight sites were sampled. The study focuses on the petrographic and microtextural characteristics of the beachrocks to elucidate their cementation environment and diagenetic evolution. The results revealed the predominance of three main cement types: (1) fibrous aragonite cement with morphologies including isopachous rims around grains with or without micritic substrates, pore-filling radial aggregates, pseudospherulite and randomly oriented interlocking aragonite fibers; (2) microcrystalline low-Mg calcite (LMC) cement binding grains and filling inter- and intra-granular pore spaces; and (3) micritic cement in the form of envelopes and/or pore-filling micrite in some samples with meniscus bridges. The last two types also contain infiltrated silt-sized clasts and skeletal remains. The cements indicate precipitation mainly in pore-filled marine phreatic diagenetic zone and, rarely, in the marine vadose zone with the aid of microbial and algal activity. Petrographic investigations and energy-dispersive X-ray spectroscopy of selected samples showed the occurrences of aragonite and LMC.

Diagenesis of Holocene Beachrock in Northeastern Brazil: Petrology, Isotopic Evidence and Age

This work aims to understand beachrock formation during Holocene through petrology, geochemistry and dating of a core located 5,43m deep in relation to present-day sea-level in the Piedade Beach in Jaboatao dos Guararapes- PE. The core is located 143 meters distant from the coast line towards mainland. Four lithofacies have been identified, taking into account differences in texture and sedimentary structures. The framework grain size ranges from medium to very coarse sand, indicating variations in depositional energy. Petrographic data indicate that the beachrocks have values of 64.23% to 70.69% of framework composed of quartz grains. Carbonate cement (represents 13.5%) consisting of high magnesium calcite surrounding grains as isopachous fringe (4.15% to 11.95%), micritic cement (0.60% to 6.42%) and equant cement (0.26% to 7.01%), indicates marine environment precipitation. The results of the isotopic composition of carbon and oxygen present valuesof 3.09‰ to 3.89‰, with an average of 3.63‰ to δ 13 C PDB and of -0.91‰ to 0.96‰, with an average of 0.54‰ to δ 18 O PDB, suggesting that the cement is formed in a shallow marine environment with freshwater influence. The values obtained in paleotemperature vary from 21oC to 30oC, with an average of 23oC, with little variation indicating precipitation in a shallow water environment. In the study area, the beachrocks are indicators of sea level and the results suggest that between 7.509 years B.P. and 5982 years B.P. there was a marine transgression process, with sea level 9.18 m lower than the current sea level.

Early Triassic stromatolites from the Xingyi area, Guizhou Province, southwest China: geobiological features and environmental implications

An Early Triassic stromatolitic deposit is documented in the Dienerian succession of the Lower Triassic Feixianguan Formation in the Xingyi area, Guizhou, southwest China. Five types of constructional microbial forms at various scales were observed. (1) Typical stratified columnar structures, up to 25 cm in height, with crinkled laminae. Dark-coloured laminae, 1 mm thick, are composed of upright filamentous tubes, average diameter 5 µm, showing a vertical growth fabric. (2) Prostrate filaments showing strong fluorescence, in sharp contrast to the micritic cement. (3) Coccoid-like spheroids and algal filaments are also common in stromatolitic laminae. These resemble present-day cyanobacteria, and thus may represent fossilized cyanobacteria. (4) Smaller bacilli resembling Pelodictyon are very common in the stromatolitic laminae. (5) Framboidal pyrite is also abundant and probably indicates biological involvement in stromatolite formation. Two major microbial functional groups, oxygenic phototrophs represented by lithified cyanobacteria and probable sulfate-reducing bacteria represented by framboidal pyrite, were present during stromatolite growth. Another possible microbial functional group, anoxygenic phototrophs represented by lithified remains resembling Pelodictyon clathratiforme, may be present in the Xingyi stromatolites, and were involved in stromatolite formation by capturing or adhering microcrystalline particles. All of these features demonstrate that the Early Triassic stromatolites are biogenic. The occurrence of the Xingyi stromatolites, corresponding to a second episode of microbial growth during the Early Triassic, reveals that post-extinction microbialites were widespread in the Dienerian. These Early Triassic stromatolites indicate that a microbial bloom took place in the aftermath of the Permian–Triassic mass extinction.

Petrography, lithofacies and depositional environment of carbonate-clastic strata in part of the north-western Douala Basin, Cameroon

The lithofacies, petrofacies and sedimentary environments of carbonate-siliciclastic strata of the Tertiary Nkapa Formation have been examined. This paper provides new data on the carbonates whose location and composite thicknesses of outcrop lithologies represent an important segment of the broader West-Central African Platform. The main carbonate outcropping belt is along the Bongoue River and its tributaries. Field and petrographic investigations reveal that carbonates in the study area consist of a number of lithologically distinct facies. These facies are non-skeletal to skeletal in composition including olithic, perlithic, bioclastic and marly limestones which occur in alternation with sandstones and shales. The studied limestones were mostly grain-supported exhibiting textures that range from grainstone, packstone to wackestone. Non-skeletal bioclasts are mainly made up of superficial and composite ooids, and peloids showing grainstone and packstone texture, respectively. The ooids have grain diameter varying from 0.1 to 1.5 mm. The pellets are of faecal origin. The bioclastic limestones are characterised with a packstone to wackestone texture (fifty–fifty), typically associated with nodules and abundant skeletal fragments suggesting deposition in shallow marine barrier reef settings. This interpretation is further supported by the presence of worm tubes in the limestones. Micritic and sparry calcite cement and opaque grains (FeS2) make up the matrix and infill within skeletal fragments. The limestones range from abundant biosparites to minor biomicrites.The marly limestones are characterised with a wacke-to-packstone texture, mainly made up of abundant lithoclasts and minor bioclasts indicating their deposition in areas closer to the continent with large influx of terrigenous clastic material. Sandstones are mostly sublitharenites (>75 %) with minor litharenites (≤25 %). The different lithofacies (carbonates, sandstones, shales) investigated demonstrate that these sediments were deposited in a wide range of environments that varies from continental shelve zones, shallow marine (reef related) to relatively deep marine environments.

Coupled micromorphological and stable isotope analysis of Quaternary calcrete development

Pedogenic calcretes are widespread in arid and semi-arid regions. Using calcrete profiles from four river terraces of the Rio Alias in southeast Spain, this study explores the potential of using detailed micromorphological and stable isotopic analysis to more fully understand the impacts of Quaternary environmental change on calcrete development. The four profiles increase in carbonate complexity with progressive age, reflecting calcretisation over multiple glacial–interglacial cycles since MIS 9 (c. 300ka). Calcrete profiles contain a mixture of Alpha (non-biogenic) and Beta (biogenic) microfabrics. Alpha fabrics have higher δ13C and δ18O values. The profiles contain a range of crystal textures, but there is little difference between the δ13C and δ18O values of spar, microspar, and micrite cements. Strong positive covariance between δ13C and δ18O suggests that both isotopes are responding to the same environmental parameter, which is inferred to be relative aridity. The study reveals that the detailed co-analysis of calcrete micromorphology and stable isotope signatures can allow patterns of calcrete formation to be placed into a wider palaeoclimatic context. This demonstrates the potential of this technique to more reliably constrain the palaeoenvironmental significance of secondary carbonates in dryland settings where other proxy records may be poorly preserved.

A NEW MODEL FOR THE FORMATION OF DOLOMITE IN THE TRIASSIC DOLOMITES, NORTHERN ITALY

The Pale di San Martino and Pale di San Lucano (referred to together as the “Pale”) are remnants of an originally more extensive carbonate platform in the Dolomite Mountains of northern Italy. The platforms are composed of Middle Triassic dolomites and limestones up to 1.6km thick. Limestones comprise 2–3% of the platform carbonates and are restricted to narrow corridors (tens to a few hundred metres wide, hundreds of metres long and high) within the dolomite. The mainly sucrosic dolomites of the Pale are interpreted as the result of recrystallization of a depositional, nearly stoichiometric Mg calcite under burial temperatures of ca. 40–70°C. The principal arguments are: The quantitative composition indicates that all platform carbonates are composed mainly of micritic crusts (45%; boundstone fabric prevails) and early cement (35%; microcrystalline, fibrous). The platform carbonates were probably mainly bacterial precipitates and tight at the sediment-water interface (porosities <5%, permeabilities in the micro-Darcy range). The limestone-dolomite transitions (centimetres to decimetres wide) lack dolomite gradients. The lack of evidence for flowing fluids causing dolomitization suggests stagnant pore waters. The δ13C of average dolomite is 1.3‰ heavier than that of coeval limestone (666 analyses). The difference corresponds to a primary difference of 50mol% MgCO3 and is interpreted as the result of fractionation. It suggests a dolomite precursor of very high Mg calcite, whereas present-day limestone of the Pale was probably deposited as a basically Mg-free polymorph (aragonite and/or calcite). The dolomite δ18O (+1 to −11‰ VPDB) values show a scatter over the platform thickness and preserve randomly distributed values around 0‰. The scatter is probably due to selective re-setting of δ18O near pore spaces and is mainly a sampling effect. The observation that 87Sr/86Sr ratios (77 analyses) of limestone and dolomite are either slightly higher or lower than Middle Triassic seawater, but almost never “normal marine”, suggests that the platform carbonates of the Pale were deposited from seawater contaminated with artesian freshwater. The limestone corridors are probably caused by artesian springs of somewhat higher than ambient depositional temperature, with low Mg calcite and/or aragonite deposited in or near fracture zones. The volumetrically subordinate cycle-cap dolomite is possibly a primary precipitate.

Geologia e paleontologia do biohermito da Formação Pirabas (Mioceno Inferior)

O presente trabalho compreende o estudo geológico e paleontológico detalhado da litofácies recifal da Formação Pirabas, Estado do Pará, aflorante na Praia do Maçarico, município de Salinópolis, como um corpo pequeno, isolado e lenticular, que deve ter se desenvolvido na rampa interna da plataforma do Mar de Pirabas. O conteúdo biótico é rico e variado, composto por três grupos de algas coralíneas, poríferos, corais, briozoários, equinoides, foraminíferos, ostracodes, e moluscos. Foram reconhecidos neste recife em mancha três estágios de sucessão ecológica, estabilização, colonização e diversificação, onde as finas crostas micríticas reconhecidas na parte superior do estágio diversificação indicam degradação das condições ambientais favoráveis à acreção recifal, provavelmente devido a continentalização do sítio deposicional. O exame petrográfico revelou apenas uma microfácies deposicional, biomicrito, sem consideráveis variações texturais e composicionais. A cimentação marinha é predominante, representada por massas microcristalinas e cimentos micríticos, localmente neomorfisados para bioesparito, e detritos carbonáticos advindos do fluxo biogênico inconsolidado, resultantes principalmente da dispersão de cristais de aragonita dos fragmentos algálicos, acumulados sob a forma de lama micrítica. Para a gênese da bioconstrução as algas foram fundamentais na consolidação e modelagem do arcabouço, principalmente as macroalgas vermelhas, decisivas para a proteção e cimentação dos demais organismos.

Porosity gain and loss in unconventional reservoirs: Example of rock typing in Lower Cretaceous hemipelagic limestones, SE France (Provence)

We characterize porosity evolution in hemipelagic limestones of the Lower Cretaceous Vocontian Basin (Northern Provence, SE France), which are surface analogs for deep unconventional reservoirs. Firstly, a structural and sedimentologic analysis was conducted on outcrops and on borehole cores of Berriasian to Lower Valanginian rocks. Secondly, 330 plugs were analyzed for facies, pore typing, porosity and acoustic Vp velocity measurements. This petrophysical dataset was compared to the burial-uplift history of the host rock.Hemipelagic carbonates include four sedimentary formations comprising nine facies splitted into mud- and grain-dominated formations which display 0–8% porosity range depending on the initial grain to mud ratio of the sediments and on basin stress regime evolution during burial-uplift history. Initial porosity was better preserved in grain dominated formations where early micritic cements built bridges between grains, generating a solid skeleton that prevented/inhibited the burial compaction effects. In mud dominated facies, micrite was initially diffuse within the rock matrix but was progressively rearranged under burial stress, while the generation of an extensive amount of stylolites occurred. The differential response of compacted grain and mud dominated facies to a late regional 1 km uplift resulted in large porosity generation (should range from 15 to 35%) localized in the mud dominated facies through the dilatancy of stylolite and pressure-solution seams.This study highlights how important is the combination of classical approaches including facies, pore type and diagenesis characterization with the structural analyses. One key result applicable to several types of unconventional reservoirs is the effective porosity generated by reactivation of compaction structures.

Weathering of carbonate materials in ancient Maya constructions (Río Bec and Dzibanché): Limestone and stucco deterioration patterns

Carbonate materials – limestone and lime stuccos – were widely used in ancient Maya construction. Deterioration of these materials presents a major problem for the conservation of Maya architectural heritage. However, the mechanisms of deterioration and their interaction with the diverse inner structures of these materials are still insufficiently understood. We performed micromorphological and mineralogical research of the limestone alteration in the buildings of Río Bec and stuccos in Dzibanché, two Late Classic Maya sites in the southern Yucatan peninsula. Dissolution and recrystallization of calcite were found to be universal and the most important weathering process in both materials. Dissolution is selective, affecting mostly micrite. Larger crystals, especially rhombohedral calcitic pseudomorphs developed after dolomite, are more stable. Selective dissolution of micritic cement of limestones and stuccoes enhances physical breakdown by producing numerous pores and loosening bounds between coarser inclusions. This process clearly controls the deterioration grade of different limestones: those with more intensive fragmentation and powdering have higher contents of micrite. Secondary carbonates are represented mostly by acicular calcite filling fissures and forming coatings on the outer surface. Clear spatial relation to fungi and roots and their similarity to well-known soil neoformations indicate biogenic origin of these neoformations. In most cases, they contribute to macroscopic powdering being loose. However, in the cracks of some limestones they form dense infillings, which could plug the fractures and prevent further advance of weathering to the core of the stone. Neoformed sulfates and iron oxides are fewer, but also participate in the deterioration.